Japanese laid-open patent publication No. 116118 of 1996, relates to an optical fiber amplifier and more particularly to an amplifying system where pumping light is sent from an LD element into an Er doped optical fiber via a signal light channel to produce an excited state in the abovementioned Er doped optical fiber. By inputting the signal light into the optical fiber amplifier and causing the light to pass through the Er doped optical fiber, the signal light is amplified and is outputted.
In order to generate pumping light sent into an Er doped optical fiber in an optical fiber amplifier, a laser diode (hereinafter merely called LD) element is used.
In recent years, on the basis of requests for shortening the work time in assembling optical fiber amplifiers and downsizing of mounting areas, several optical elements including an LD element are in advance integrated in one package to make an integrated module for an optical fiber amplifier, which would be incorporated in an optical fiber amplifier.
As an example of an integrated module for a conventional optical fiber amplifier, a so-called backward pumping type, in which pumping light is sent into an Er doped optical fiber in reverse of the signal light advancing direction is shown in FIG. 4(a) and FIG. 4(b).
As shown in FIG. 4(a) and FIG. 4(b), in the integrated module for an optical fiber amplifier, the respective optical elements (WDM filter component 5, optical isolator 6, beam splitter component 7) are mounted in a package consisting of a substrate 1 and side plates 2a through 2d erected at four sides of the substrate 1.
Sealing glass components 3a and 3b are, respectively, secured at the side plates 2a and 2b, wherein the first optical fiber 4a (signal light inputting portion) which inputs signal light into the abovementioned module and sends pumping light from the module to an Er doped optical fiber (not illustrated) is fixed outside one sealing glass component 3a, and the second optical fiber 4b (signal light outputting portion) which outputs the abovementioned signal light from the corresponding module is fixed outside the other sealing glass component 3b.
These first and second optical fibers 4a, 4b are disposed so that the end faces thereof are confronting each other, and a signal light channel X through which signal light goes, is formed from the optical fiber 4a to the optical fiber 4b via above the substrate 1.
Furthermore, a lens (not illustrated) which collimates light is provided between the sealing glass component 3a and optical fiber 4a and between the sealing glass component 3b and optical fiber 4b.
WDM filter component 5, optical isolator 6 and beam splitter component 7 are disposed and fixed at the part which is made into the signal light channel X on the substrate 1, after their beam axes are aligned with each other.
For example, as shown in FIG. 5, the WDM filter component 5 and beam splitter component 7 are such that they are accommodated and fixed, by cementing with low melting point glass or welding by a YAG laser or soldering, at a metal holder 8 consisting of Fe-Ni-Co based alloy (hereinafter called KOVAR), 42Ni-Fe (42 alloy) or stainless steel, etc., and they are directly fixed on the substrate 1 below the underside of the metal holder 8 by laser beam welding such as YAG laser welding, etc.
An LD element 9 which generates pumping light is disposed at the side of the WDM filter component 5.
A heat sink which quickly absorbs heat generated at the LD element 9 is fixed on the underside of the LD element 9, and a base 11 consisting of, for example, Cu or Cu-W based alloy is fixed on the underside of the heat sink, and a Peltier element 12 is attached to the underside of the base 11. Furthermore, the underside of the Peltier element 12 is fixed on the substrate 1 by brazing such as soldering or Ag brazing, etc.
Furthermore, 13 is a collimator lens which collimates pumping light emitted from the LD element 9.
Photo diodes 14,14 (hereinafter merely called PD) are disposed at both sides of the beam splitter component 7. These PDs 14,14 are, respectively, attached to the side plates 2c, 2d.
Furthermore, the integrated module for optical fiber amplifier is sealed by a substrate 1, side plates 2a through 2d and upper face plate 17 in a state where nitrogen, etc., is internally enclosed, and is fixed at an optical fiber amplifier substrate by fixing with screws at, for example, an opening portion 15 of the substrate 1.
In such an integrated module for an optical fiber amplifier, signal light advances in the channel X in FIG. 4(a).
That is, signal light is inputted from the first optical fiber 4a into the module via the sealing glass component 3a and is made incident into the second optical fiber 4b end face via the sealing glass component 3b, passing from the first optical fiber 4a end face through the sealing glass component 3a, WDM filter component 5, optical isolator 6, and beam splitter component 7, wherein the signal light is outputted from the second optical fiber 4b to outside of the module.
Furthermore, a part of the signal light is reflected outside the signal light channel X by the beam splitter component 7 and is sampled by one side PD 14. Furthermore, in the beam splitter component 7, reflection light reversely advancing from the second optical fiber 4b into the signal light channel X is reflected in the reverse direction of the case of the signal light, and is sampled by the other PD 14.
Furthermore, the pumping light advances in a channel Y in FIG. 4(a).
That is, the pumping light is emitted from the LD element 9, reflected by the WDM filter component 5, made incident into the end face of the first optical fiber 4a, and is outputted from the first optical fiber 4a to outside the module.
Accordingly, the pumping light is provided into an Er doped optical fiber (not illustrated) and contributes to producing an excited state of the Er doped optical fiber.
Furthermore, the optical isolator 6 is an optical component to allow light to pass through in one direction and is able to interrupt the reflection light advancing from the second optical fiber 4b to the first optical fiber 4a side.
Herein, as a means for fixing optical components such as a WDM filter component 5, a beam splitter component 7, etc., on the substrate 1, laser beam welding by YAG welding, etc., which is able to firmly fix these components for a longer period of time, is used.
However, laser beam welding is a method by which heat is concentrated at the boundary of attached members to be welded and both members are instantaneously welded to each other. However, if the thermal conductivity of the members to be welded is high, heat is diffused via the members, wherein the welding itself is not carried out in a satisfactory condition.
Therefore, in an integrated module for a conventional optical fiber amplifier, in order to firmly fix the respective optical components such as a WDM filter component 5, etc., on the substrate 1 by laser beam welding, etc. for a longer period of time, a substrate made of KOVAR and stainless steel, which has a low thermal conductivity and is obtained at a low cost, were used as a substrate 1.
However, in such an integrated module for an optical fiber amplifier, it is required that the temperature of the LD element 9 is adjusted by the abovementioned Peltier element 12, in order to secure and maintain the laser characteristics of the LD element 9, and it is highly important that the output and reliability of the LD element 9 are further increased and improved in line with a demand for high output of signal light.
In order to meet such a demand, it is necessary to efficiently conduct heat, which is generated at the lower part of the Peltier element 12 when adjusting the temperature of the Peltier element 12, to the substrate 1 and to diffuse heat from the substrate 1 to outside the module.
However, conventionally, the thermal conductivity of materials such as KOVAR, stainless steel, etc., which are used for a substrate 1 is, for example, 30 W/mK or less, and is considerably low. Therefore, in the abovementioned module, it is difficult to efficiently diffuse heat, which is generated at the lower part of the Peltier element 12 when adjusting the temperature of Peltier element 12, outside the module.
Accordingly, in order to solve these problems, it is considered that the material of a substrate 1, which is adhered to and fixed at the lower part of Peltier element 12 by soldering, ,etc., is changed to a material, for example, CU and Cu-W based alloy, which has a better thermal conductivity than that of the abovementioned KOVAR usually used, thereby causing the thermal diffusion performance to outside the module to be improved.
Actually however, the problem which arises at this point is laser beam welding which is used to fix optical components such as the abovementioned WDM filter component 5, etc., to the substrate 1.
That is, since the thermal conductivity of CU or Cu-W based alloy is too high if a substrate 1 made of Cu or Cu-W based alloy is welded to a metallic holder 8 part of an optical component by laser beam welding, the heat due to the laser beam welding was quickly diffused, and the welding property was not of a satisfactory quality.
Therefore, it was difficult to firmly fix the substrate 1 and optical component such as a WDM filter component 5, etc., for a longer period of time.
Furthermore, when fixing the integrated module for optical fiber amplifiers to a substrate for optical fiber amplifiers, there are cases where deformation such as warping of the substrate 1 arises at the integrated module for optical fiber amplifiers.
At this time, the beam axes of the LD element 9 and the WDM filter component 5, the axes of which have been aligned when producing a module, deviated from each other to cause the optical coupling ratio to be lowered. That is, such a problem arises, by which the light quantity of the pumping light emitted from the first optical fiber 4a to outside the module is greatly decreased.
The present invention was developed in order to solve the abovementioned problems and other shortcomings, and it is therefore an object of the invention to provide an integrated module for optical fiber amplifiers, which is able to efficiently emit heat by a Peltier element outside the module, has a WDM filter component and a substrate firmly fixed for a longer period of time, and is able to minimize the beam axis deviation between an LD element and a WDM filter component even though deformation arises at the substrate.